Microscopia de varrimento por sonda (SPM)

Gazeta da Física • VOL. 18 • Fascículo 4

Molecular tectonics : supramolecular 2D nanopatterning of surfaces by self-assembly

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Influence of heat treatment on the optical properties of SnO2 : Sb thin films deposited by dip coating using aqueous solution

Antimony doped tin oxide thin films were deposited on glass by a chemical route derived from Pechini method. Particular emphasis was given to the microstructure of crystallized films. Crystalline phase formation was studied by grazing incident X-ray diffraction and by thermal analyses. Scanning electron microscopy was carried out for microstructure characterization, surface roughness was observed using scanning tunneling microscope and the optical transmittance measurements were performed in the wavelength range of 200-800 nm. (C) 2002 Kluwer Academic Publishers.

Ab initio studies of pristine and oxidized Cu3Au(100) and (111) surfaces

We review the previous literature and our recent work on first-principles studies of Cu3Au(100) and (111) surfaces, with focus on the segregation of atomic species to the surface at pristine conditions and in the presence of oxygen. In particular, the combined use of experimental and theoretical tools to achieve chemical identification at an atomic level of the surface species is emphasized and discussed.

Synthesis and Supramolecular Architectures of Lipophilic Derivatives of Guanine

Self-assembly relies on the association of pre-programmed building blocks through non-covalent interactions to give complex supramolecular architectures. Previous studies provided evidence for the unique self-assembly properties of semi-synthetic lipophilic guanosine derivatives which can sequestrate ions from an aqueous phase, carry them into an organic phase where they promote the generation of well-defined supramolecular assemblies. In the presence of cations lipophilic guanosines form columnar aggregates while in their absence they generate supramolecular ribbons. The aim of this thesis has been the synthesis of guanine derivatives, in particular N9-alkylated guanines and a guanosine functionalized as a perchlorotriphenylmetil moiety (Gace-a-HPTM) in order to observe their supramolecular behaviour in the absence of sugar (ribose or deoxyribose) and in the presence of a bulky and chiral substituent respectively. By using guanine instead of guanosine, while maintaining all the hydrogen bond acceptor and donor groups required for supramolecular aggregation, the steric hindrance to supramolecular aggregation is notably reduced because (i.e. guanines with groups in N9 different from sugar are expected to have a greatest conformational freedom even in presence of bulky groups in C8). Supramolecular self-assembly of these derivatives has been accomplished in solutions by NMR and CD spectroscopy and on surface by STM technique. In analogy with other guanosine derivatives, also N9-substituted guanines and GAceHPTM form either ribbon-like aggregates or cation-templated G-quartet based columnar structures.

Classification of suppressor additives based on synergistic and antagonistic ensemble effects

Three fundamental types of suppressor additives for copper electroplating could be identified by means of potential Transient measurements. These suppressor additives differ in their synergistic and antagonistic interplay with anions that are chemisorbed on the metallic copper surface during electrodeposition. In addition these suppressor chemistries reveal different barrier properties with respect to cupric ions and plating additives (Cl, SPS). While the type-I suppressor selectively forms efficient barriers for copper inter-diffusion on chloride-terminated electrode surfaces we identified a type-II suppressor that interacts non-selectively with any kind of anions chemisorbed on copper (chloride, sulfate, sulfonate). Type-I suppressors are vital for the superconformal copper growth mode in Damascene processing and show an antagonistic interaction with SPS (Bis-Sodium-Sulfopropyl-Disulfide) which involves the deactivation of this suppressor chemistry. This suppressor deactivation is rationalized in terms of compositional changes in the layer of the chemisorbed anions due to the competition of chloride and MPS (Mercaptopropane Sulfonic Acid) for adsorption sites on the metallic copper surface. MPS is the product of the dissociative SPS adsorption within the preexisting chloride matrix on the copper surface. The non-selectivity in the adsorption behavior of the type-II suppressor is rationalized in terms of anion/cation pairing effects of the poly-cationic suppressor and the anion-modified copper substrate. Atomic-scale insights into the competitive Cl/MPS adsorption are gained from in situ STM (Scanning Tunneling Microscopy) using single crystalline copper surfaces as model substrates. Type-III suppressors are a third class of suppressors. In case of type-land type-II suppressor chemistries the resulting steady-state deposition conditions are completely independent on the particular succession of additive adsorption. In contrast to that a strong dependence of the suppressing capabilities on the sequence of additive adsorption ("first comes, first serves" principle) is observed for the type-IIIsuppressor. This behavior:is explained by a suppressor barrier that impedes not only the copper inter-diffusion but also the transport of other additives (e.g. SPS) to the copper surface. (C) 2011 Elsevier Ltd. All rights reserved.

Single-Molecule Junctions Based on Nitrile-Terminated Biphenyls: A Promising New Anchoring Group

We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.

Nanoscale imaging of chemical interactions: Fluorine on graphite

Using C60-functionalized scanning tunneling microscope tips, we have investigated the adsorption of fluorine on graphite. Based on characteristics of the accompanying electron standing waves, we are able to distinguish the fluorine adatoms that have bonded ionically to the graphite surface from those that have formed covalent bonds with the surface. This result permits determination of the ratio of ionic to covalent C–F bonds on graphite obtained by gas phase fluorination, which seems to be temperatureindependent between 200 and 300°C under the reaction conditions used.

Fabrication and characterization of metallic nanowires

The shape of metallic constrictions of nanoscopic dimensions (necks) formed using a scanning tunneling microscope is shown to depend on the fabrication procedure. Submitting the neck to repeated plastic deformation cycles makes it possible to obtain long necks or nanowires. Point-contact spectroscopy results show that these long necks are quite crystalline, indicating that the repeated cycles of plastic deformation act as a “mechanical annealing” of the neck.

Mechanical, electrical, and magnetic properties of Ni nanocontacts

The dynamic deformation upon stretching of Ni nanowires as those formed with mechanically controllable break junctions or with a scanning tunneling microscope is studied both experimentally and theoretically. Molecular dynamics simulations of the breaking process are performed. In addition, and in order to compare with experiments, we also compute the transport properties in the last stages before failure using the first-principles implementation of Landauer's formalism included in our transport package ALACANT.

Spin-transfer torque on a single magnetic adatom

We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunneling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunneling. By changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunneling conductance.

Topologically protected quantum transport in locally exfoliated bismuth at room temperature

We report electrical conductance measurements of Bi nanocontacts created by repeated tip-surface indentation using a scanning tunneling microscope at temperatures of 4 and 300 K. As a function of the elongation of the nanocontact, we measure robust, tens of nanometers long plateaus of conductance G0=2e2/h at room temperature. This observation can be accounted for by the mechanical exfoliation of a Bi(111) bilayer, a predicted quantum spin Hall (QSH) insulator, in the retracing process following a tip-surface contact. The formation of the bilayer is further supported by the additional observation of conductance steps below G0 before breakup at both temperatures. Our finding provides the first experimental evidence of the possibility of mechanical exfoliation of Bi bilayers, the existence of the QSH phase in a two-dimensional crystal, and, most importantly, the observation of the QSH phase at room temperature.

Design and Implementation of Tools for STM Studies in UHV and at the Solid-Liquid Interface

This thesis presents details of the design and development of novel tools and instruments for scanning tunneling microscopy (STM), and may be considered as a repository for several years' worth of development work. The author presents design goals and implementations for two microscopes. First, a novel Pan-type STM was built that could be operated in an ambient environment as a liquid-phase STM. Unique features of this microscope include a unibody frame, for increased microscope rigidity, a novel slider component with large Z-range, a unique wiring scheme and damping mechanism, and a removable liquid cell. The microscope exhibits a high level of mechanical isolation at the tunnel junction, and operates excellently as an ambient tool. Experiments in liquid are on-going. Simultaneously, the author worked on designs for a novel low temperature, ultra-high vacuum (LT-UHV) instrument, and these are presented as well. A novel stick-slip vertical coarse approach motor was designed and built. To gauge the performance of the motor, an in situ motion sensing apparatus was implemented, which could measure the step size of the motor to high precision. A new driving circuit for stick-slip inertial motors is also presented, that o ffers improved performance over our previous driving circuit, at a fraction of the cost. The circuit was shown to increase step size performance by 25%. Finally, a horizontal sample stage was implemented in this microscope. The build of this UHV instrument is currently being fi nalized. In conjunction with the above design projects, the author was involved in a collaborative project characterizing N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on Au(111) films. STM was used to characterize Au substrate quality, for both commercial substrates and those manufactured via a unique atomic layer deposition (ALD) process by collaborators. Ambient and UHV STM was then also used to characterize the NHC/Au(111) films themselves, and several key properties of these films are discussed. During this study, the author discovered an unexpected surface contaminant, and details of this are also presented. Finally, two models are presented for the nature of the NHC-Au(111) surface interaction based on the observed film properties, and some preliminary theoretical work by collaborators is presented.

Preparació de puntes de tungstè per a microscòpia d'efecte túnel (STM)

Es presenten els resultats experimentals obtinguts durant l’estudi sistemàtic realitzat de la preparació electroquímica de puntes de tungstè per al Microscopi d’Efecte Túnel (STM), fent servir dos electròlits: KOH i NaOH. L’estudi sobre la morfologia, longitud de la punta i radi de curvatura de la punta en funció del voltatge aplicat i les concentracions de l’electròlit es descriu al capítol 3. La caracterització de les puntes es va dur a terme, per una part, mitjançant un microscòpic electrònic de rastreig (SEM) i per l’altre banda, amb el ús de les puntes obtingudes al STM. En resumen, els resultats mostren que ambdós electròlits permeten obtenir puntes que es poden fer servir amb èxit per l’obtenció d’imatges amb l’STM. Les millors puntes són aquelles que s’obtenen dins de rangs de concentracions d’electròlit baixes, entre valor de 10 a 15% en pes pel NaOH i entre 10 i 20% pel KOH i rangs de voltatge entre 3 a 7 V pel NaOH i 4 a 8 V pel KOH. S’observa que es requereixen temps d’atac electroquímic menors fent servir com a electròlit NaOH. S’estudia, en el capítol 4, el tractament que requereix la punta per tal d’eliminar les impureses de la seva superfície. Es realitzen diferents proves amb tres mètodes de neteja: (1) tractament químic, (2) bombardeig iònic i (3) tractament tèrmic de recuit. En el capítol 5 del projecte s’analitzen les imatges d’una mostra d’or, Au(110), d’estructura coneguda, amb el microscopi d’efecte túnel STM) del laboratori fent servir les puntes obtingudes sota les condicions considerades òptimes. El resultat confirma el bon comportament de les puntes obtingudes sota les condicions descrites en els capítols anteriors i establert una pauta a seguir per obtenir puntes d’una manera senzilla i reproduïble.

MgO/Ag(001) interface structure and STM images from first principles

The interface of MgO/Ag(001) has been studied with density functional theory applied to slabs. We have found that regular MgO films show a small adhesion to the silver substrate, the binding can be increased in off-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of O atoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanning tunneling microscopy signatures of these films is also analyzed in some detail. For defect free deposits containing 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at large positive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxide they introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of the image, the extra O stored at the interface can also be detected for very thin films.